Water Electrolytic system

ABSTRACT

A water electrolytic system includes a water electrolyzer, a photovoltaic generator which is a power source for the water electrolyzer, and a DC/DC converter adapted to convert the maximum output from the photovoltaic generator into a current and a voltage corresponding to an IV characteristic of the water electrolyzer by converting current and voltage for a portion of such maximum output, and to input the converted current and voltage to the water electrolyzer. With the water electrolytic system, even when an optimal point of operation of the photovoltaic generator, i.e., the maximum output from the photovoltaic generator has been varied, the efficient operation of the water electrolytic system can be carried out utilizing such maximum output.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a water electrolytic system designed so that a water electrolyzer is operated by an output from a photovoltaic generator.

[0003] 2. Description of the Related Art

[0004] A photovoltaic generator has an optimal point of operation, namely, an operating current and an operating voltage at the time when the output from the photovoltaic generator assumes a maximum value. If the optimal point of operation is matched with an IV characteristic (I: current, V: voltage) of a water electrolyzer, the water electrolyzer can be operated with a good efficiency. However, the optimal point of operation is varied depending on the temperature of the photovoltaic generator, an insolation amount to the photovoltaic generator and the like and as a result, the optimal point of operation is not matched with the IV characteristic of the water electrolyzer. When the photovoltaic generator and the water electrolyzer have been connected in series to each other, namely, connected directly to each other, it is difficult to operate the water electrolytic system with a good efficiency at all times.

[0005] Therefore, in order to ensure that even when the optimal point of operation of the photovoltaic generator, i.e., the maximum output from the photovoltaic generator, has been varied, the water electrolyzer is operated efficiently by utilizing such maximum output, a water electrolytic system has been developed (for example, see Japanese Patent Application Laid-open No. 7-233493), which includes a water electrolyzer, a photovoltaic generator, and a high-output type DC/DC converter adapted to convert all of the maximum output from the photovoltaic generator into a current and a voltage corresponding to an IV characteristic of the water electrolyzer to input them to the water electrolyzer.

[0006] However, the high-output type DC/DC converter is large in size with a weight of about 60 kg and moreover, is of a high cost and has an efficiency of 80 to 90% and hence, a loss of 10 to 20% is produced. For this reason, the conventional system has a problem that it lacks in economy.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an object of the present invention to provide an economical water electrolytic system designed so that even when the optimal point of operation of the photovoltaic generator, i.e., the maximum output from the photovoltaic generator has been varied, the water electrolyzer can be operated with a good efficiency by utilizing such maximum output.

[0008] To achieve the above object, according to the present invention, there is provided a water electrolytic system comprising a water electrolyzer, a photovoltaic generator which is a power source for the water electrolyzer, and a DC/DC converter adapted to convert the maximum output from the photovoltaic generator into a current and a voltage corresponding to an IV characteristic of the water electrolyzer by carrying out the conversion of current and voltage for a portion of the maximum output and to input the current and the voltage to the water electrolyzer.

[0009] With such arrangement, when the maximum output from the photovoltaic generator has been varied, the water electrolyzer can be operated with a good efficiency by utilizing the varied maximum output. Moreover, the conversion of current and voltage is carried out for a portion of the maximum output from the photovoltaic generator by the DC/DC converter and hence, a loss in the entire system can be suppressed to a small level, and a low-output type of a DC/DC converter made in a small size and at a low cost can be employed, leading to an increase in economy.

[0010] According to the present invention, there is provided a water electrolytic system comprising a water electrolyzer, a photovoltaic generator which is a power source for the water electrolyzer, and a DC power source adapted to add an external output to the maximum output from the photovoltaic generator in order to provide a current and a voltage corresponding to an IV characteristic of the water electrolyzer.

[0011] With such arrangement, when the maximum output from the photovoltaic generator has been varied, a new optimal point of operation corresponding to such variation can be allowed to appear by adding an external output corresponding to the variation to the varied maximum output, and can be matched with the IV characteristic of the water electrolyzer and determined as a point of operation for the water electrolyzer. Thus, the water electrolytic system can be operated with a good efficiency.

[0012] The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a graph showing a first example of an IV characteristic for a photovoltaic generator and a water electrolyzer;

[0014]FIG. 2A is a diagram of an electric circuit of example 1 of a first embodiment of the present invention;

[0015]FIG. 2B is an equivalent circuit of the electric circuit shown in FIG. 2A;

[0016]FIG. 3 is a graph showing a second example of the IV characteristic for a photovoltaic generator and a water electrolyzer;

[0017]FIG. 4A is a diagram of an electric circuit of example 2 of the first embodiment of the present invention;

[0018]FIG. 4B is an equivalent circuit of the electric circuit shown in FIG. 4A;

[0019]FIG. 5 is a diagram of an electric circuit of example 1 of a second embodiment of the present invention;

[0020]FIG. 6 is a graph showing a third example of the IV characteristic for a photovoltaic generator and a water electrolyzer; and

[0021]FIG. 7 is a diagram of an electric circuit of example 2 of the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] FIGS. 1 to 4B show a first embodiment of a water electrolytic system 1. The first embodiment includes example 1 and example 2. FIGS. 1, 2A and 2B show example 1, and FIGS. 3, 4A and 4B show example 2.

[0023] In example 1, FIG. 1 shows a mode when the maximum output from a photovoltaic generator 2 has been varied. A line L1 indicates an IV characteristic of the photovoltaic generator 2 at 28° C. (an insolation amount: 1040 W/m²), and a line L2 indicates an output characteristic of the photovoltaic generator 2. In this case, an optimal point P1 of operation of the photovoltaic generator 2 is a point at which the output is a maximum value. Such maximum output Emax is 1506 W. On the other hand, a line L3 indicates an IV characteristic of a water electrolyzer (number of water electrolytic cells: 8) 4. When the photovoltaic generator 2 and the water electrolyzer 4 has been connected in series, a point of intersection between the line L1 indicating the IV characteristic of the photovoltaic generator 2 and the line L3 indicating the IV characteristic of the water electrolyzer 4 is a point P2 of operation of the water electrolyzer 4. This point P2 of operation is at a location displaced from the optimal point P1 of operation of the photovoltaic generator 2 toward a lower current and higher voltage side and hence, the highly efficient operation of the water electrolytic system 1 cannot be desired.

[0024] To deal with this, in example 1, a peak power tracking is carried out, which comprises controlling a low-output type DC/DC converter 5, so that the operating current Imax at the optimal point P1 of operation of the photovoltaic generator 2 is decreased, and the operating voltage Vmax is increased, measuring a current Imax−dI and a voltage Vmax+dV lying on the line L3 indicating the IV characteristic of the water electrolyzer 4, and determining these current and voltage as an operating current and an operating voltage for the water electrolyzer 4.

[0025] As in FIG. 2A, in example 1, the low-output type DC/DC converter 5 is connected in series between the photovoltaic generator 2 and the water electrolyzer 4. The low-output type DC/DC converter 5 has a function to conduct the conversion of current and voltage for a portion of the maximum output from the photovoltaic generator 2, thereby converting the maximum output into a current and a voltage corresponding to the IV characteristic of the water electrolyzer 4 and inputting the current and the voltage to the water electrolyzer 4, in order to carry out the peak power tracking. FIG. 2B shows an equivalent circuit shown in FIG. 2A.

[0026] To operate the water electrolyzer 4 under a situation shown in FIG. 1, the operating current Imax (92.4 A) and the operating voltage Vmax (16.3 V) at the optimal point P1 of operation of the photovoltaic generator 2 are first determined. When the photovoltaic generator 2 and the water electrolyzer 4 have been connected in series to each other, the point P2 of operation of the water electrolyzer 4 is a point at which the operating current is 80 A and the operating voltage is 17.8 V.

[0027] When the low-output type DC/DC converter 5 is then operated to decrease the resistance value of a variable resistor R, as shown in FIG. 2B, a current dI flows. Therefore, the current to the water electrolyzer 4 is decreased into Imax−dI, while the voltage is increased into Vmax+dV. This is continued to measure a current Imax−dI (73.5 A) and a voltage Vmax+dV (17.7 V) lying on the line L3 indicating the IV characteristic of the water electrolyzer 4. A point P3 having these current and voltage is determined as a point of operation of the water electrolyzer 4.

[0028] Namely, an output of (Vmax+dV).(Imax−dI) is supplied from the photovoltaic generator 2 to the water electrolyzer 4. In this case, a portion Vmax.dI of the maximum output Imax.Vmax from the photovoltaic generator 2 has been converted into (Imax−dI).dV at the supplied output (Vmax+dV).(Imax−dI), wherein (Imax−dI).dV is a value resulting from the subtraction of a loss due to the conversion from Vmax.dI. In this case, the weight of the low-output type DC/DC converter 5 was about 8 kg, which is about 13% of the weight of a high-output type DC/DC converter, and the efficiency was equal to or higher than 94%.

[0029] In example 2, FIG. 3 shows a mode when the maximum output from the photovoltaic generator 2 has been varied. A line L1 indicates an IV characteristic of the photovoltaic generator 2 at 28° C. (an insolation amount: 1040 W/m²), and a line L2 indicates an output characteristic of the photovoltaic generator 2, as in example 1. In this case, In this case, an optimal point P1 of operation of the photovoltaic generator 2 is a point at which the output is a maximum value. Such maximum output Emax is 1506 W. On the other hand, a line L3 indicates an IV characteristic of a water electrolyzer (number of water electrolytic cells: 6) 4.

[0030] When the photovoltaic generator 2 and the water electrolyzer 4 has been connected in series, a point of intersection between the line L1 indicating the IV characteristic of the photovoltaic generator 2 and the line L3 indicating the IV characteristic of the water-electrolyzing device 4 is a point P2 of operation of the water electrolyzer 4. This point P2 of operation is at a location displaced from the optimal point P1 of operation of the photovoltaic generator 2 toward a higher current and lower voltage side and hence, the highly efficient operation of the water electrolytic system 1 cannot be desired.

[0031] To deal with this, in example 2, a peak power tracking is carried out, which comprises controlling a low-output type DC/DC converter 5, so that the operating current Imax at the optimal point P1 of operation of the photovoltaic generator 2 is increased, and the operating voltage Vmax is decreased, measuring a current Imax+dI and a voltage Vmax−dV lying on the line L3 indicating the IV characteristic of the water electrolyzer 4, and determining these current and voltage as an operating current and an operating voltage for the water electrolyzer 4.

[0032] As in FIG. 4A, in example 2, the low-output type DC/DC converter 5 is connected in parallel between the photovoltaic generator 2 and the water electrolyzer 4. The low-output type DC/DC converter 5 has a function to conduct the conversion of current and voltage for a portion of the maximum output from the photovoltaic generator 2, thereby converting the maximum output into a current and a voltage corresponding to the IV characteristic of the water electrolyzer 4 and inputting the current and the voltage to the water electrolyzer 4, in order to carry out the peak power tracking. FIG. 4B shows an equivalent circuit shown in FIG. 4A.

[0033] To operate the water electrolyzer 4 under a situation shown in FIG. 3, the operating current Imax (92.4 A) and the operating voltage Vmax (16.3 V) at the optimal point P1 of operation of the photovoltaic generator 2 are first determined. When the photovoltaic generator 2 and the water electrolyzer 4 have been connected in series to each other, the point P2 of operation of the water electrolyzer 4 is a point at which the operating current is 100 A and the operating voltage is 13.8 V.

[0034] When the low-output type DC/DC converter 5 is then operated to increase the resistance value of a variable resistor R, as shown in FIG. 2B, a current dI flows. Therefore, the current to the water electrolyzer 4 is decreased into Imax+dI, while the voltage is increased into Vmax−dV. This is continued to measure a current Imax+dI (108.2 A) and a voltage Vmax−dV (13.9 V) lying on the line L3 indicating the IV characteristic of the water electrolyzer 4. A point P3 having these current and voltage is determined as a point of operation of the water electrolyzer 4.

[0035] Namely, an output of (Vmax−dV).(Imax+dI) is supplied from the photovoltaic generator 2 to the water electrolyzer 4. In this case, a portion Imax.dV of the maximum output Imax.Vmax from the photovoltaic generator 2 has been converted into (Vmax−dV).dI at the supplied output (Vmax−dV).(Imax+dI), wherein (Vmax−dV).dI is a value resulting from the subtraction of a loss due to the conversion from Imax.dV.

[0036] FIGS. 5 to 7 show a second embodiment of the present invention. In example 1 of the second embodiment shown in FIG. 5, a DC power source 6 is connected in series between a photovoltaic generator and a water electrolyzer 4. The DC power source 6 has a function to add an external output to the maximum output from the photovoltaic generator 2 in order to provide a current and a voltage corresponding to the IV characteristic of the water electrolyzer 4.

[0037] In FIG. 6, a curve L1 indicates an IV characteristic of the photovoltaic generator 2 at 80° C. (an insolation amount: 905 W/m²), and a line L2 indicates an output characteristic of the photovoltaic generator 2. An optimal point of operation of the photovoltaic generator 2 is a point at which a relative output assumes a maximum value; an operating voltage is 13.3 V, and an operating current is 92 A (1224 W). On the other hand, a line L3 indicates an IV characteristic of the water electrolyzer (number of water electrolytic cells: 7) 4, and a line L4 indicates a power characteristic of the water electrolyzer 4.

[0038] When the photovoltaic generator 2 and the water electrolyzer 4 have been connected in sires to each other, a point of intersection between the line L1 indicating the IV characteristic of the photovoltaic generator 2 and the line L3 indicating the IV characteristic of the water electrolyzer 4, namely, a point of a lower current and higher voltage than those at the optimal point of operation, is a point P2 of operation of the water electrolyzer 4.

[0039] Therefore, when the optimal point P1 of operation is shifted toward a higher voltage side in a state of a constant current (92 A), the optimal point P1 is in accord with the line L3 indicating the IV characteristic of the water electrolyzer 4 at a voltage of 15.6 V. An operating voltage at the optimal point P1 of operation is 13.3 V, and 15.6 V−13.3 V=2.3 V. Therefore, when an external output of 2.3 V from the DC power source 6 is added to the IV characteristic of the photovoltaic generator 2, a secondary IV characteristic provided by cooperation of the photovoltaic generator 2 and the DC power source 6 with each other is as indicated by a line L5 shown by a dotted line, and a new optimal point of operation in such IV characteristic is a point of operation of the water electrolyzer 4.

[0040] When the maximum output from the photovoltaic generator 2 has been varied, as described above, a new optimal point P3 of operation corresponding to such variation, i.e., a point having an operating voltage of 15.6 V and an operating current of 92 A (1435 W) can be allowed to appear by adding an external output corresponding to such variation to the varied maximum output. Such optimal point P3 can be matched with the IV characteristic of the water electrolyzer 4 and determined as a point of operation for the water electrolyzer 4. The water electrolyzer 4 can be operated with a good efficiency by such peak power tracking.

[0041] In this case, a power of 1435 W corresponding to the optimal point P3 of operation in the water electrolyzer 4 is indicated by a point P4 on the line L4. This power is a value provided by increasing, by 47%, a power of 976 W shown by a point P5 on the line L4 before addition of the external output in the water electrolyzer 4. The details of the increase rate of 47% are as follows: If the maximum output from the photovoltaic generator 2 shown by a point P6 on the line L2 is 1223 W, a component A of the increase rate provided by a peak power tracking effect is 25.3%, and a component B provided by the external output is 21.7%. In this case, the efficiency is 94%.

[0042] In example 2 of the second embodiment shown in FIG. 7, a DC power source 6 is connected in parallel between as a photovoltaic generator 2 and a water electrolyzer 4. Even in example 2, a peak power tracking can be carried out. 

What is claimed is:
 1. A water electrolytic system comprising a water electrolyzer, a photovoltaic generator which is a power source for said water electrolyzer, and a DC/DC converter adapted to convert the maximum output from said photovoltaic generator into a current and a voltage corresponding to an IV characteristic of said water electrolyzer by converting current and voltage for a portion of said maximum output, and to input the converted current and voltage to said water electrolyzer.
 2. A water electrolytic system comprising a water electrolyzer, a photovoltaic generator which is a power source for said water electrolyzer, and a DC power source adapted to add an external output to the maximum output from said photovoltaic generator in order to provide a current and a voltage corresponding to an IV characteristic of said water electrolyzer. 